Cure rates

Several synthetic pyrimidines and purines are useful drugs Acyclovir was the first effective antiviral compound and is used to treat herpes infections 6 Mercaptopunne is one of the drugs used to treat childhood leukemia which has become a very treatable form of cancer with a cure rate approaching 80%... 

C]-Urea Cure characteristics Cured meat Cured silicone LIM Cure rate... 

The utihty of these adhesives arises from the electron-withdrawing character of the groups adjacent to the polymerizable double bond, which accounts for both the extremely high reactivity or cure rate and thek polar nature, which enables the polymers to adhere tenaciously to many diverse substrates. 

Carbon black grades identified by four characters (ASTM D1765-67), ie, cure rate of normal (N) or slow (S), digit classifyiag typical particle size ia am, and two arbitrarily assigned characters. 

High ortho novolaks have faster cure rates with hexa. Typical properties of a 2inc acetate-cataly2ed high ortho novolak are also shown in Table 4. The gel time with hexa is one-third of that with a strong acid-cataly2ed novolak. 

Toluhydroquinone and methyl / fX butyUiydroquinone provide improved resin color retention 2,5-di-/-butyIhydroquinone also moderates the cure rate of the resin. Quaternary ammonium compounds, such as benzyl trimethyl ammonium hydroxide, are effective stabilizers in combination with hydroquinones and also produce beneficial improvements in color when promoted with cobalt octoate. Copper naphthenate is an active stabilizer at levels of 10 ppm at higher levels (150 ppm) it infiuences the cure rate. Tertiary butylcatechol (TBC) is a popular stabilizer used by fabricators to adjust room temperature gelation characteristics. 

Fig. 5. Influence of catalyst systems on cure rate gelation time is at 25°C as a function of the initiator concentration. A represents MEKP (1.0%) B, MEKP...

The action of redox metal promoters with MEKP appears to be highly specific. Cobalt salts appear to be a unique component of commercial redox systems, although vanadium appears to provide similar activity with MEKP. Cobalt activity can be supplemented by potassium and 2inc naphthenates in systems requiring low cured resin color lithium and lead naphthenates also act in a similar role. Quaternary ammonium salts (14) and tertiary amines accelerate the reaction rate of redox catalyst systems. The tertiary amines form beneficial complexes with the cobalt promoters, faciUtating the transition to the lower oxidation state. Copper naphthenate exerts a unique influence over cure rate in redox systems and is used widely to delay cure and reduce exotherm development during the cross-linking reaction. 

Eor apphcation temperatures below 10°C or for acceleration of cure rates at room temperature, nonredox systems such as ben2oyl peroxide initiated by tertiary amines such as dimethylaruline (DMA) have been appHed widely. Even more efficient cures can be achieved using dimethyl- -toluidine (DMPT), whereas moderated cures can be achieved with diethylaruline (DEA). 

Tertiary amines are also effective as accelerators in cobalt redox systems to advance the cure rate (Eig. 6). Hardness development measured by Shore D or Barcol D634-1 penetrometer can be used to demonstrate this benefit, which is useful in increasing mold turnover at ambient temperatures. 

Eig. 2. Cure rate of PPS as a function of soHd-state cure temperature. 

Fig. 10. Generalized formulation design outline for radiation-curable coatings and adhesive systems. The cross-linker is a multifimctional unsaturated cross-linking agent or oligomer, rj = viscosity CR = cure rate S = shrinl ge H = hardness F = flexibility A = adhesion 7 = surface energy ...

Clinically, GM-CSF or G-CSF have been used to accelerate recovery after chemotherapy and total body or extended field irradiation, situations that cause neutropenia and decreased platelets, and possibly lead to fatal septic infection or diffuse hemorrhage, respectively. G-CSF and GM-CSF reproducibly decrease the period of granulocytopenia, the number of infectious episodes, and the length of hospitalization in such patients (152), although it is not clear that dose escalation of the cytotoxic agent and increased cure rate can be rehably achieved. One aspect of the effects of G-CSF and GM-CSF is that these agents can activate mature cells to function more efficiently. This may, however, also lead to the production of cytokines, such as TNF- a, that have some toxic side effects. In general, both cytokines are reasonably well tolerated. The side effect profile of G-CSF is more favorable than that of GM-CSF. Medullary bone pain is the only common toxicity. 

Ethylene—Propylene Rubber. Ethylene and propjiene copolymerize to produce a wide range of elastomeric and thermoplastic products. Often a third monomer such dicyclopentadiene, hexadiene, or ethylene norbomene is incorporated at 2—12% into the polymer backbone and leads to the designation ethylene—propylene—diene monomer (EPDM) mbber (see Elastomers, synthetic-ethylene-propylene-diene rubber). The third monomer introduces sites of unsaturation that allow vulcanization by conventional sulfur cures. At high levels of third monomer it is possible to achieve cure rates that are equivalent to conventional mbbers such as SBR and PBD. Ethylene—propylene mbber (EPR) requires peroxide vulcanization. 

Fig. 2. Cure curve from oscillating disk rheometer where A represents scorch safety B, cure rate C, state of cure D, optimum cure time and E, reversion.

Meta.1 Oxides. Halogen-containing elastomers such as polychloropreae and chlorosulfonated polyethylene are cross-linked by their reaction with metal oxides, typically ziac oxide. The metal oxide reacts with halogen groups ia the polymer to produce an active iatermediate which then reacts further to produce carbon—carbon cross-links. Ziac chloride is Hberated as a by-product and it serves as an autocatalyst for this reaction. Magnesium oxide is typically used with ZnCl to control the cure rate and minimize premature cross-linking (scorch). 

There are seven principal classes of accelerators and several miscellaneous products that do not fit into these classes. In addition, many proprietary blends of several accelerators are sold which are designed as cure packages for a specific appHcations. Choosing the best cure system is a responsibiUty of the mbber chemist and requites extensive knowledge of each accelerator type and its appHcabiUty in each elastomer. Table 5 shows a rule of thumb comparison of the scorch/cure rate attributes for the five most widely used classes of accelerators used in the high volume diene-based elastomers. 

Accelerator type Scorch safety Cure rate Cro ss-link length... 

As a general rule the sulfenamides exhibit faster cure rate than the thiazoles. If secondary accelerators are used, dithiocarbamates are scorchiest and give the fastest cure followed by the thiurams, then the guanidines. Figure 6 summarizes these comparisons to show a series of natural mbber (NR) recipes using either a thiazole (MBTS) or sulfenamide (TBBS) primary accelerator in combination with the various secondary accelerators (21). In this study, the initial primary accelerator levels were selected to produce nearly equivalent modulus or state of cure in the NR. 

Natural mbber usually contains sufficient levels of naturally occurring fatty acids to solubilize the zinc salt. However, if these fatty acids are first extracted by acetone, the resultant "clean" natural mbber exhibits a much lower state of cure. Therefore, to ensure consistent cure rate, fatty acids are usually added. Synthetic mbbers, especially the solution polymers, do not contain fatty acids and requite thein addition to the cure system. 

Fig. 7. Effect of activators on cure rate where A is 2.5 phr sulfur B, sulfur + stearic acid (2 phr) + zinc oxide (5 phr) C, sulfur + TBBS (0.6 phr) D, sulfur + TBBS + stearic acid E, sulfur + TBBS + zinc oxide and E, sulfur + TBBS + stearic acid + zinc oxide. To convert cm /kg to in./lb, divide by 5.5.

The thiophthalimide (CTP) and sulfenamide classes of retarders differ from the organic acid types by thek abiUty to retard scorch (onset of vulcanization) without significantly affecting cure rate or performance properties. Much has been pubUshed on the mechanism of CTP retardation. It functions particularly well with sulfenamide-accelerated diene polymers, typically those used in the the industry. During the initial stages of vulcanization, sulfenamides decompose to form mercaptobenzothiazole (MBT) and an amine. The MBT formed reacts with additional sulfenamide to complete the vulcanization process. If the MBT initially formed is removed as soon as it forms, vulcanization does not occur. It is the role of CTP to remove MBT as it forms. The retardation effect is linear with CTP concentration and allows for excellent control of scorch behavior. 

In one experiment the effect of ppd assay was correlated to scorch safety. As the ppd degrades Hberate free amine, scorch time decreases and cure rate is faster. The degradation products apparentiy serve to activate the cure, since both the induction time, and cure time, decrease with decreasing ppd assay. However, the effect on unaged properties is minimal. 

Fillers. Materials used as fillers (qv) in mbber can also be classified as acidic, basic, or neutral. Furnace blacks, ie, HAF, FEF, or SRF, are somewhat basic. As such, they can have an activating effect on sulfur cure rates. Furthermore, carbon blacks have been found to promote formation of mono/disulfide cross-links thereby helping minimize reversion and enhance aging properties. 

Channel blacks such as the old EPC and MPC grades are acidic to neutral and can vary from having Httie effect to having a slight retardation in cure rate. The pH-neutral large-particle size MT thermal black generally has Httie effect on cure rate. 

To assist in control of the onset of vulcanization, a retarder or prevulcanization iuhibiter (PVI) is used. Retardation of the onset of cure does not mean that the rate of cure is slowed, in fact cure rate may actually be increased. Rather, there is an induction period prior to cure. 

Cure rate Property No antioxidant Antioxidant, 1.0 part (diy)... 

This is an activator-starved formulation and so is highly sensitive to the presence of nonmbbers that are capable of activating or accelerating vulcanization, and Table 2 illustrates the cure behavior of different grades of SMR (28). Cup lump grades show the highest state of cure and fastest rate of cure, whereas the stabilized grade, SMR CV, shows the lowest state of cure and slowest cure rate. 

The cure rate of a sihcone sealant is dependent on the reactivity of the cross-linker, catalyst type, catalyst level, the diffusion of moisture into the sealant, and the diffusion of the leaving group out of the sealant. For one-part sealants, moisture diffusion is the controlling step and causes a cured skin to form on the exposed sealant surface and progress inward. The diffusion of moisture is highly dependent on the temperature and relative humidity conditions. 

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